The long-term goal of our research is to understand the cellular mechanisms by which precisely organized auditory networks become established and how this process is influenced by normal and abnormal neuronal activity. The proposed project is aimed at elucidating these mechanisms in a tonotopically organized, inhibitory sound localization pathway in the auditory brainstem. Specifically, we aim to shed light on the mechanisms by which the inhibitory connections from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) become tonotopically organized and functionally fine-tuned. We recently demonstrated that formation of the tonotopic organization of the MMTB-LSO pathway involves specific functional elimination (silencing) and strengthening of GABA/glycinergic connections before hearing onset. We also found that during this period of reorganization, MNTB-LSO synapses can undergo activity- dependent long-term depression that involves cannabinoid signaling and, in addition to GABA and glycine, surprisingly also release glutamate as a third neurotransmitter. Building on these findings we are proposing experiments to a) illuminate the basic cellular and synaptic mechanisms by which MNTB-LSO connections become strengthened or eliminated during circuit reorganization and b) shed light on the seemingly paradoxical phenomenon of glutamate release from GABA/glycinergic MNTB-LSO synapses. To achieve these goals we will apply a variety of modern electrophysiological and imaging techniques, such as whole-cell and perforated patch clamp recordings, confocal calcium imaging, and mapping of functional connectivity with focal photolysis of caged glutamate, in auditory brainstem slices prepared from neonatal mice. The proposed research may provide new insights into the basic rules and activity-dependent mechanisms by which primary sound localization circuits in the mammalian brain become assembled and functionally fine- tuned. Detailed knowledge of these mechanisms is crucial for understanding the emergence of abnormal auditory circuits that result from early cochlea damage or malfunction and thus can help us to understand the cause of human communication disorders such as language perception, specific language impairment, and dyslexia that result from impaired auditory processing and that likely have developmental components.